Keyboards are used in a wide variety of state-of-the-art equipment as the interface between a human operator and a system, or machine, into which data, control signals, or the like are entered. Using supplementing logic circuitry, keyboards may be conveniently employed to translate keyed-in information into codes which are intelligible to the system, as for example in a data processing system with which the keyboard interfaces. Keyboards are frequently used for this purpose in the increasingly popular distributed data processing systems which employ free-standing terminals for interactive data processing.
At the present state of the data processing art, large numbers of circuits may be formed on a single semiconductor chip. Hence all signals, including keyboard-originated signals, must be provided at compatibly low amplitudes and compatibly high pulse frequencies. At these low amplitudes it is often advantageous to employ a capacitive keyboard wherein the actuation of the key changes the capacitance of a circuit rather than producing a switch closure. The increased capacitance which is provided upon key actuation supplies the necessary capacitive coupling for transmitting an applied signal through the circuit. The presence of a coupled-through signal at the output then indicates that the key was actuated, e.g. depressed to its active state.
This technique requires that the applied signal be prevented from being coupled through when the key is in its inactive state. Thus, means must be provided to detect the presence of the signal as well as its absence. In view of the fact that some capacitive coupling exists even when the key is inactive, the verification of key actuation will depend on the ability of the circuit to discriminate between two relatively small signals, rather than between the presence and absence of a signal. Hence, the signal detection means must be very sensitive and it is therefore highly susceptible to interference by ambient electromagnetic noise. When it is considered that the capacitance in the active and the inactive key states may be on the order of 40 and 20 picofarads respectively, it will be readily apparent that spurious noise signals could be interpreted as having resulted from the actuation of the key. An incorrect verification of key actuation or de-actuation, will result in the transmission of false data to the interfacing system. Such a situation is particularly likely to arise when a key is actuated for an extended period of time. For example, a key may be locked in the acutated position in order to print capital letters only. Since the key is periodically tested, the presence of noise under those conditions may be interpreted as a large number of separate key actuations.
Likewise, false data may be transmitted if there is any ambiguity concerning key actuation during the travel of the key between its active and inactive positions. Specifically, since the capacitance varies from a minimum to a maximum between these two positions, at some point during the travel of the key weak signal coupling will occur and key actuation will be verified upon testing. However, the slightest noise on the line may provide a false reading during the subsequent key test. Since these tests occur in close succession, the key may still be moving toward its active position. The problem described assumes added significance when it is considered that the operator may not fully depress a particular key, particularly when the keys are rapidly actuated in succession.
The problem of noise interference has been attacked in a number of ways in prior art keyboards, with varying degrees of success. A brute force approach calls for massive shielding to screen out noise. Noise interference has also been minimized by judiciously routing the conductors along predetermined paths and by assigning specific physical key locations in the electronic scanning sequence.
Prior art solutions to the problem of "key teasing", i.e. the ambiguity which occurs when the key is in an intermediate position, also take various forms. Snap action keyboard switches have been used which trip to the fully actuated position after being pressed to the halfway point of key travel. Another approach calls for boosting the amplitude of the coupled-through pulse signal of those keys that have been previously declared to be in the active position.
While these approaches have enjoyed some measure of success, this has been achieved at a significant cost increase and in many instances by an increase in the complexity of the keyboard. Further, such enhancement of the reliability of verification as has been obtained, has usually been limited to verifying key actuation, but not for verifying key switching to the inactive state.